1. Field of the Invention
The present invention generally relates to switch keypads and, more particularly, to an improved rubber keypad useful for cellular portable telephones.
2. Discussion of Prior Art
With the proliferation of portable cellular telephones, there has been an increasing need for telephone keypads which have the individual keys illuminated in some fashion to aid in their operation. An additional necessity of such a keypad is a means for providing a tactile feedback which is an indication that the particular key has been actuated.
In the past, relatively high-profile keypads incorporating plunger-type actuating keys have been utilized to permit back illumination as well as good tactile feedback. Generally, such high-profile keys utilize a carbon conductor or a metallic popple dome rendering back illumination difficult. Such problems have been solved in part by using a separate light pipe (see U.S. Pat. No. 4,124,879) or a thick rubber pad itself as a light pipe (see U.S. Pat. No. 4,636,593) for distribution of required illumination to the keys of the keypad. However, because the thickness of such high-profile keypads requires a substantial increase in the overall dimensions of the cellular portable telephone system, it is desirable to provide a low-profile, rubber keypad with a similar degree of illuminability.
U.S. Pat. No. 4,916,262 is an example of such a low profile, rubber keypad which permits illumination to pass through a diffuser plate and through various translucent layers of the keypad. However, such a construction suffers from poor simultaneity of tactile feedback and electrical contact. Further, the illumination levels required in order to provide acceptable illumination of the key itself requires an unacceptable power drain during operation.
A further problem with U.S. Pat. No. 4,916,262 is the existence of a separate shorting layer requiring a further adhesive layer in order to bond the shorting layer in the appropriate interrelationship. The shorting layer, and its necessary adhesive layer, add to the thickness of what is desired to be a low-profile keypad.
It has been proposed to utilize light emitting diodes which provide the desired level of illumination with an acceptable power drain. In order to utilize the light generated by the light emitting diode (LED), it is generally felt necessary to mount the LED directly under the key of the keypad. However, in this instance, the height of the light emitting diode above the printed circuit board upon which it is mounted serves to interfere with operation of the conventional tactile feedback dome unless the dome is spaced a substantial vertical distance from the light emitting diode. Accordingly, the use of LED illuminated rubber keypads has in the past precluded the manufacturer of a low-profile keypad.